Electrical apparatus for loudspeaker systems



ec. 7, W48. w. o. CRANE 2,455,471

ELECTRICAL APPARATUS FOR LOUD srmpn SYSTEMS Fiid March 28, 1946 PatentedDec. 7, 1948 ELECTRICAL APPARATUS FOR LOUD- SPEAKER SYSTEMS Webster 0.Crane, St. Charles, 111., assignor to Operadio Manufacturing 00., St.Charles, 11].,

a corporation of Illinois Application March 28, 1946, Serial No. 657,881

9 Claims. I

This invention relates to an electrical apparatus and particularly to aspeaker system. In public address systems, announcing systems or alarmsystems, it is desirable to operate a number of speakers and to vary thenumber of such speakers in accordance with load requirements. If aspeaker system is to be operated efi'iciently, it becomes important thatproper matching between the output transformer of the system and thespeakers be maintained. While a certain amount of mis-match may betolerated, the eiiiciency of the systemis greatly reduced. if themagnitude of the speaker load varies through substantial limits. Unlessmeans are provided for introducing some degree of regulation, the audiofrequency potential available across speakers may rise sharply withtheremoval of a substantial number of speakers from the load. The converseis also true.

It has hitherto been the practice to provide resistor as dummy loads forspeakers. In addition to wasting power, the resistors frequently burnout.

To overcome the disadvantage incident to dummy resistor loads, the arthas relied upon scalied parallel feedback transformers for endowing aspeaker system with satisfactory regulation. This parallel feedbacktransformer comprises a three-winding transformer having one windingwhich may be arbitrarily considered as a primary and the remaining twowindings which may be considered as secondaries. The primary winding inthis system is connected in shunt to the speaker load. The secondarywindings are connected in the input circuits of a push-pull amplifier.The phasing between primary and secondaries is such as to providedegeneration. Thus, as the potential across the speaker load increasesabove a predetermined normal value, degeneration will increase and therewill be anincreasing tendency to cut down the signal strength at theinput of the amplifiers. Conversely, if the potential across the speakerload tends to decrease toward a-. predetermined normal value, thedegeneration becomes weaker; It is understood that the potentialsreferred to'are audio'frequency potentials. Thus, when the'syst'em isoperating under what might be termed anarbitrary normal load condition,a predetermined amount of degeneration occurs in the feedbacktransformer. The regulation depends upon the variation in thisdegenerative action.

In the systemdescribed above, it has been found that the regulationpermits substantial variation in the potential applied to-the speakersresulting in variation in the power output from individual speakers asthe load changed. Asa rule, it has been found that a power variation ofnot less than three db. at each speaker would result from a change ofspeaker load in spite of the regulation. Thus, the regulating action ofthe parallel feedback transformer inherently requires a substantialpotential variation across the speaker load for operation.

Since a three db. power variation represents a substantial change, asfar as a human ear is concerned, it is desirable to improve theregulation. The invention herein provides a system wherein the potentialacross the speaker loads is maintained substantially constant over widevariations of speaker load. The invention provides, in one respect, athree-winding feedback transformer wherein the primary is connected inseries with the entire speaker load rather than in shunt thereto. Byvirtue of such a connection, the variation of current through the loadis relied upon for regulation. This three-winding transformer, which maybe considered as a series feedback transformer, has the primary andsecondaries so phased that the action is regenerative.

To control regeneration, minimize instability and control frequencynon-discrimination, the primary may be by-passed with a suitableresistor. In order to reduce distortion introduced by a regenerativefeedback transformer, the invention additionally provides for theinclusion in the system of a parallel feedback transformer connected asin the prior art. Thus, the parallel feedback transformer secondariesare connected in series with the secondaries of the series feedbacktransformer. By virtue of the degenerative action of the parallelfeedback transformer, the amount of regeneration in the series feedbacktransformer may be increased above what might be considered desirable ifa parallel feedback transformer is omitted. The combination of the twofeedback transformers provides a compound feedback action which isparticularly effective as far as regulation is concerned and provides aminimum of distortion.

Referring, therefore, to the drawing, the single figure shows a circuitdiagram of a multi-speaker system embodying the present invention. l0represents any suitable source of audio frequency as a microphone,phonograph pick-up or other sound reproducing system, or that portion ofa radio receiver which provides audio frequencies. Source ID; which initself may include as many stages of amplification" as desired, isconnected to primary II of driver transformer l2. Transl4 prol5 andformer l2 has secondary windings l3 and viding a common groundedconnection outer terminals 8 and 9.

Transformer l2 feeds its output to series feedback transformer I1consisting of primary l8 and secondaries I9 and 20. Transformer i1 is aconventional iron core audio frequency transformer with the variouswindings closely coupled to each other. The phasing of the windings areregenerative as will be indicated later.

Outer terminals 8 and 9 of driver transformer I2 are connectedrespectively to secondary windings l9 and 2D. secondaries l9 and areconnected to secondaries 22 and 23 respectively of transformer 24similar to transformer 11. Windings 22 and 23 extend the circuit tocontrol grids 26 and 21 of push-pull amplifiers 28 and 29 respectively.These amplifiers are power amplifiers and, for simplicity, are shown asthree electrode tubes. It is understood, however, that other amplifiers,such as beam power tubes, may be used.

Amplifiers 28 and 29 have cathodes 3t and 3| grounded at 32. These tubeshave anodes 34 and 35 connected to primaries 36 and 31 respectively ofpush-pull output transformer 38. Primary windings 36 and 31 have centertap 39 for connection to the usual source of B plus supply. Outputtransformer 38 has secondary 40 across which condenser 4| is connected.Condenser 4! serves to stabilize against oscillation. Secondary 4B andcondenser 41 together have terminals 42 and 43 respectively. Terminal 42is connected by line 44 back to grounded primary I8 of transformer I1.Across primary [8, there is preferably disposed low resistance 45 forcontrolling regeneration and frequency non-discrimination. Thisresistance may be omitted if desired under certain conditions. Terminal43 is connected by lead 46 back to grounded primary 41 of transformer24.

Connected between terminal 43 and ground are speakers 50 of any desiredconstruction. Thus, if these speakers are of the permanent magnet type,the speaker input will be connected between terminal 43 and ground. Ifthese speakers are of electro-dynamic type, it will be understood thatthe voice coils are connected between terminal 43 and ground and thatother means for supplying current to the field windings will have to beprovided. Whether or not the field circuit in a dynamic speaker iscut-out together with the voice coil makes little difference with regardto the invention. Suitable switching means are diagrammaticallyindicated by switches in the speaker leads.

It will be noted that primary l8 and resistor 45 is in series with theentire speaker load. Transformer l1 may be considered as aseries-connected feedback transformer.

Transformer primary 41 of transformer 24, on the other hand, isconnected across terminal 43 and ground so that it is effectively inshunt to all the speaker loads. This transformer may, therefore, beconsidered as a parallel feedback transformer.

Referring now to transformer l1, primary I8 is so phased with referenceto the two secondaries that the transformer action is regenerative forthe entire system. Thus, any signal going through primary l8 will inducein both secondaries l9 and 20 potentials tending to increase the signalstrength already in the secondaries by virtue of the output from drivertransformer windings l3 and 14. By shunting primary IS with a res tance,the amount of regeneration may be determined while keeping the over-allresistance of the output circuit for secondary 40 low.

Primary 41 on the other hand is phased with reference to secondaries 22and 23 so that degenerative action occurs. Thus, a signal throughprimary 41 will induce potentials in secondaries 22 and 23 which tend tooppose the normal signal potentials in these secondaries by virtue oftheir connection in the output circuit of windings l3 and I4. The amountof degeneration in transformer 24 may be controlled by the transformerratio or by disposing a resistance in series with primary winding 41. Iftransformer 24 is omitted, series feedback transformer I1 may providegood regulation but tends to introduce distortion. The distortion, as arule, increases with the amount of regeneration. By decreasingregeneration in transformer I1, however, regulating action isundesirably affected.

By combining both transformer l1 and 24, as shown in the system, thedistortion which may arise by virtue of regeneration is reduced greatlyby the opposing degeneration introduced by transformer 24. In fact, byhaving both transformers, substantial regeneration in transformer l1 maybe tolerated without rendering the entire system unstable andsusceptible to oscillation.

The combination of transformer l1 and 24 is highly effective inproviding a regulating characteristic which is effective for slightchanges in output from secondary 40. As is well known, the essence ofregulation resides in the utilization of slight variations in thequantity being regulated to maintain the quantity being regulatedconstant within limits. Thus, as speakers are added or subtracted fromthe load, little or no variation in potential across the speaker andlarge variation in load current suffice to introduce a regulating actionin the amplifier circuit.

The impedance as seen by output winding 40 remains constant over a widerange of speaker loads. The impedances introduced into the output systemby primary windings l8 and 21 are complex and variable and, in aproperly designed system, tend to compensate for the variation ofimpedance due to adding or subtracting speakers. The regenerative actionin transformer 11 results in the introduction of a negative resistancein the system. The reverse is true of transformer 7.4. Thus, as speakersare added or subtracted, the relative magnitudes of these resistancesvary and maintain the efficiency of the entire system at a high levelover large variations of load.

The large variation in current drawn by the speaker load causessubstantial action in transformer l1. Any slight variation in potentialacross the speaker load causes some action in transformer 24.

By proper design of the system, various regulating characteristics maybe obtained. Thus, a constant potential across the speaker load may beobtained with wide changes in the load. It is also possible to obtain arising potential characteristic across the speaker load with increase inspeaker load. The reverse may also be obtained.

It is possible to use ordinary single tube amplifiers instead ofpush-pull. In such case, the extra secondary windings for feedback wouldbe unnecessary.

What is claimed is:

1. A speaker system comprising a driver transformer having a primaryadapted to be energized with audio frequency potentials and having asecondary, a vacuum tube amplifier having input and output circuits, afeedback transformer having primary and secondary windings, means forconnecting the secondary windings of said driver and feedbacktransformers in series in said amplifier input circuit, an outputtransformer having primary and secondary, said output primary beingconnected in said output amplifier circuit, a plurality of parallelconnected speakers forming a load, means for connecting said load,feedback primary and output secondary in series, said feedback windingsbeing so phased as to provide regenerative action whereby the potentialacross said speaker load is maintained substantially constant in spiteof load variations.

2. The system of claim 1 wherein a by-pass resistor is connected acrossthe feedback primary.

3. A speaker system comprising a driver transformer having a primaryadapted to be energized with audio frequency potentials and having asecondary with a center tap and outer terminals for push-pull operation,a vacuum tube amplifier in push-pull having cathode, grid, and anodeelectrodes, a feedback transformer having one primary and two secondarywindings, connections for disposing a secondary feedback winding betweenone control grid and the outer terminal of the driver secondary so thatsaid two feedback secondaries are in the input circuits of said pushpullamplifier, said cathodes and center tap being connected, an outputtransformer having a primary connected to the anodes of said amplifiers,said output transformer having a secondary, a plurality of parallelconnected speakers forming a load, connections for disposing said loadand the primary of said feedback transformer in series across saidoutput secondary, said feedback transformer having the primary andsecondary windings so phased as to provide regeneration.

4. The system of claim 3 wherein a by-pass resistor is providedacrosssaid feedback primary to control said regeneration.

5. A speaker system comprising a driver transformer having a primaryadapted to be fed by audio frequency potentials and having a secondary,an audio frequency amplifier having input and output circuits, a pair offeedback transformers having primary and secondary windings, means forconnecting said driver secondary and the feedback secondaries in seriesin said amplifier input circuit, an output transformer having a primaryin the amplifier output circuit, said output transformer having asecondary, a plurality of speakers in parallel forming a load, means forconnecting one feedback primary in series with said load across saidoutput secondary and means for connecting the other feedback primaryacross said load, said one seriesconnected feedback transformer havingthe windings phased to provide regeneration and the other parallelfeedback transformer having the windings phased to provide degeneration.

6. The system of claim 5 wherein a by-pass resistor is connected acrosssaid one feedback primary in said series feedback transformer.

7. A speaker system comprising a driver transformer having a primaryadapted to be energized with audio frequency potentials and having asecondary with two outer and one center terminal, a pair of vacuum tubeamplifiers, each having cathode, grid and anode electrodes, means forconnecting said cathodes to said center terminal of said driversecondary, circuits connecting the outer driver secondary terminals tosaid control grids for push-pull operation, two feedback transformerseach having a primary and two secondaries, said grid circuits eachincluding one secondary of each transformer in series between a grid andouter terminal of the driver secondary, an output transformer having apushpull primary connected to said amplifier anodes, said outputtransformer having a secondary, a plurality of parallel connectedspeakers as a load, means for connecting the primary of one feedbacktransformer in series with said load across said output secondary andmeans for connecting the primary of the other feedback transformer inshunt across said load, said one feedback transformer having itswindings phased for regeneration and the other feedback transformerhaving its windings phased for degeneration.

8. The system of claim 7 wherein a by-pass resistor is connected acrossthe primary of said one series feedback transformer.

9. The system of claim 7 wherein a by-pass resistor is connected acrossthe primary of the one series feedback transformer and wherein acondenser is connected across the output secondary.

WEBSTER O. CRANE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,107,817 Davis Feb. 8, 19382,162,369 Campbell June 13, 1939 2,166,517 Begun July 18. 1939

